Not applicable.
The present invention relates generally to well logging tools used in oil and gas wells to characterize a subterranean formation using acoustic waves transmitted and received by the tool. More specifically, the present invention relates to methods and apparatus used to acoustically isolate the transmitter from the receiver while providing sufficient structural strength to withstand common handling and use, including pushing and pulling of the tool.
Acoustic logging tools that are used to characterize subterranean formations surrounding well bores are well known in the art. In general, acoustic logging tools operate by broadcasting an acoustic signal into a formation from one or more transmitters located at one position on the tool and receiving the signal with one or more receivers located at a second location on the tool. Properties of the received acoustic signal, such as travel time, frequency, amplitude, and attenuation, are then used to characterize the surrounding formation.
The transmitters generate a compressional waveform that travels through the borehole fluids and into the surrounding formation. The acoustic wavefields propagate through the formation in a variety of modes, the most important being compressional waves, or xe2x80x9cP-wavesxe2x80x9d, and transverse shear waves, or xe2x80x9cS-wavesxe2x80x9d. P-waves are characterized by particle motion in the direction of wave travel while S-waves are characterized by particle motion perpendicular to the direction of wave travel. The various modes of propagation are distinguishable by their relative velocities. The velocities of both P-waves and S-waves depend on the elastic constants and the density of the medium through which the waves travel.
Ideally, the only acoustic signals received by the tool""s receivers would be those signals that are transmitted by the tool that have traveled through the formation. However, if not properly isolated, the receiver will also detect other signals, sometimes referred to as xe2x80x9ctool noisexe2x80x9d or xe2x80x9croad noisexe2x80x9d. This undesired noise can interfere with the ability of the tool to render an accurate representation of the acoustic response of the formation. This noise is typically energy, more specifically vibrations, traveling within or on the surface or body of the logging tool. The noise may be a high or low frequency noise, such as that created by the transmitters or by contact of the logging tool with the wellbore. Therefore, it is desirable to isolate the receivers of a well logging tool from extraneous sources of acoustic signals, namely the transmitters and the rigid body of the tool.
U.S. Pat. No. 3,190,388, issued Jun. 22, 1965 to Moser et al., teaches a simple acoustic well logging tool with a signal attenuating structure. The logging tool comprises an outer housing made of steel and a centrally disposed bar that supports the acoustic apparatus and is connected to the outer housing at the top and bottom of the tool. To attenuate the acoustic signal, the outer housing incorporates a series of generally U-shaped, helical grooves cut into the cylindrical metal housing on both the inside and outside diameter. The grooves are of a depth greater than one-half of the wall-thickness of the housing so as to interrupt the direct travel of the signal through the housing. The grooves are arranged so that the spacing between the inner and outer grooves lengthwise of the housing is less than one quarter of the wave length of the principal frequency of the acoustic signal. The grooves may be filled with a high-density material, such as lead, to increase the weight of the housing to further inhibit acoustic transmission. An alternate method of constructing the housing using a plurality of circular openings through the wall of the outer housing is also disclosed. The acoustic transducers are mounted on the centrally mounted, hollow bar that is constructed of Teflon(trademark), or some other material with a low velocity characteristic.
U.S. Pat. No. 5,036,945, issued Aug. 6, 1991, to Hoyle et al. discloses a sonic well tool having a first and second attenuation and delay apparatus for attenuating and delaying the signal traversing the tool body. The first attenuation and delay apparatus includes interleaved rubber and metal like washers for attenuating compressional and flexural waves propagating along the body, and further includes a bellows section having a corrugated shape and a thin traverse dimension. The second attenuation and delay apparatus includes mass loading rings surrounding the housing of the well tool, and also includes a bellows section having a corrugated shape and a thin traverse dimension.
U.S. Pat. No. 5,229,553, issued Jul. 20, 1993 to Lester et al. discloses an acoustic isolator for use with a well logging tool having transducers in a first and third tool segment, which are to be acoustically isolated from receivers in a second and fourth tool segment. The acoustic isolator consists of vertebrate links composed of spools, encased by resilient boots, which spools are arranged end-to-end in tandem configuration. A plurality of split shells interconnect the spools by externally gripping the boots covering the end portions of the respective adjacent spools.
The design of acoustic isolators for downhole applications involves two requirements that are seemingly mutually exclusive. The first of these requirements is that the tool be sufficiently flexible to attenuate acoustic waves traveling at or near the surface of the tool. The second requirement is that the tool be strong enough to survive running and retrieval operations, which may be by wireline or tubing conveyed means. During these operations it is often required to push or pull heavy loads via the tool. It is desirable that this extreme loading not have any permanent deleterious effects on the performance of the isolator. Additionally, because of the nature of well logging operations, including the environment in which it occurs, it is desired to have a tool with a simple, robust design.
Typical acoustic well logging tools that incorporate acoustic isolators are long, flexible cylinders. Occasionally, well logging tools become stuck in the well bore and have to be retrieved using force. The typical acoustic isolator can not withstand the forces normally observed during these operations and recovery of the tool often results in destruction of, or major damage to, the tool. Therefore, a need exists in the art for an improved acoustic isolator capable of high rates of signal attenuation in normal logging operations and capable of withstanding high axial forces common to fishing or recovery operations.
The acoustic isolator assembly of the present invention comprises a elongated cylindrical body suited for connection to an acoustic array and subsequent disposition within a wellbore. According to one embodiment of the present invention, the acoustic isolator is a component of a well logging tool and is linearly disposed between an acoustic transmitter and an acoustic receiver. The acoustic isolator serves to restrict the propagation of acoustic signals along the length of the logging tool, therefore reducing the amount of tool noise or road noise received by the receiver.
According to one embodiment of the present invention, the acoustic isolator comprises a plurality of cylindrical isolator modules that are coaxially arranged to form the body of the tool. Each isolator module comprises a spring disposed within an outer housing. The separate isolator modules are attached to one another by connecting rods around which are disposed a plurality of nodal masses.
The isolator module further comprises mechanical stops that limit the deflection of the spring during high axial tension, or compression, loading. These features enable the acoustic isolator assembly to withstand the high loading that may be applied during logging operations. Therefore, the isolator modules are capable of supporting high compressive and tensile loads without suffering permanent deformation of the springs.
The springs are preferably encapsulated with a material having a low velocity characteristic, such as a rubber or phenolic compound. Holes may also be drilled radially through the coils of the spring to further inhibit the transmission of acoustic signals. These holes may also be filled with rubber or a similar material, as previously discussed. The outside diameter of the outer housing may be coated with a dissimilar material, such as fiberglass or another low velocity characteristic material that can withstand the wellbore environment. This coating preferably interferes with any signal traveling along the outer surface of the tool. Further, the outside surface of the tool provides an irregular surface that also aids in the attenuation of signals along the outside surface.
The acoustic isolator assembly provides a centrally located bore for electrical connection between the receiver and transmitter. The entire assembly may be filled with oil during operation. The oil prevents corrosion and eliminates any stresses from hydrostatic pressure in the wellbore.
Therefore, the acoustic isolator of the present invention effectively attenuates the higher frequency compressional waves through a series of isolator modules containing springs and attenuates the lower frequency transverse waves through the overall acoustic response of the system that is greatly improved through flexurally-limber isolator modules and nodal masses. The arrangement of the components also allows the tool to withstand high axial loading.